Monitoring shipping emissions in the German Bight using MAX-DOAS measurements

Other literature type English OPEN
Seyler, André ; Wittrock, Folkard ; Kattner, Lisa ; Mathieu-Üffing, Barbara ; Peters, Enno ; Richter, Andreas ; Schmolke, Stefan ; Burrows, John P. (2017)

A three-year time series of ground-based MAX-DOAS measurements of NO<sub>2</sub> and SO<sub>2</sub> on the island Neuwerk has been analyzed for contributions from shipping emissions. The island is located in the German Bight, close to the main shipping lane into the river Elbe towards the harbor of Hamburg. Measurements of individual ship plumes as well as of background pollution are possible from this location, which is a few kilometers from the shipping lane. A simple approach using the column amounts of the oxygen molecule dimer or collision complex, O<sub>4</sub>, for the determination of the horizontal light path length has been applied to retrieve path-averaged volume mixing ratios. An excellent agreement between mixing ratios retrieved from NO<sub>2</sub> retrievals in the UV and visible parts of the spectrum has been found, showing the validity of the approach. Obtained mixing ratios of NO<sub>2</sub> and SO<sub>2</sub> are compared to co-located in-situ measurements showing good correlation on average with good agreement for well-mixed background pollution but systematic underestimation of plume concentrations by the MAX-DOAS O<sub>4</sub> approach. Comparing data before and after the introduction of stricter fuel sulfur content limits (from 1&thinsp;% to 0.1&thinsp;%) on 1 January 2015 in the North Sea emission control area (ECA), a signifcant reduction in SO<sub>2</sub> levels has been observed. For situations with wind from the open North Sea, where ships are the only local source of air pollution, the average mixing ratio of SO<sub>2</sub> decreased by a factor of eight, while for NO<sub>2</sub> in the whole time series from 2013 till 2016 no signifcant change in emissions has been observed. More than 2000 individual ship emission plumes have been identifed in the data and analyzed for the emission ratio of SO<sub>2</sub> to NO<sub>2</sub>, yielding an average ratio of 0.3 for the years 2013/2014, decreasing signifcantly presumably due to lower fuel sulfur content in 2015/2016. By sorting measurements according to the prevailing wind direction and selecting two angular reference sectors representative for wind from open North Sea and coast excluding data with mixed air mass origin, relative contributions of ships and land-based sources to air pollution levels in the German Bight have been estimated to be around 40&thinsp;%&thinsp;:&thinsp;60% for NO<sub>2</sub> as well as SO<sub>2</sub> in 2013/2014, dropping to 14&thinsp;%&thinsp;:&thinsp;86&thinsp;% for SO<sub>2</sub> in 2015/2016.
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